Voice frequency repeaters are used in telephone communication systems to compensate for line attenuation of the audio or voice frequency signals when the line is relatively long. One typical repeater configuration separates the two directions of signal transmission (as between a central office and a subscriber's telephone) and inserts gain in each direction separately, thus achieving amplification of the signals transmitted in both directions. The present invention is concerned with this type of repeater as opposed to a negative resistance repeater which produces its gain by using a negative attenuation pad technique or the like.
To accomplish separation of the bidirectional signals for amplifying each direction separately, the electronic hybrid repeater is equipped with a pair of term (i.e., terminating) sets and a four-wire circuit connected between the two term sets. A four-wire circuit provides two lines (one of which may be common) for each direction of signal transmission, while a two-wire line provides signal transmission in both directions on the same two-wire line. A hybrid circuit in one term set couples the repeater's four-wire circuit to a two-wire circuit such as a two-wire transmission line between the repeater and the subscriber's telephone. A hybrid circuit in the other term couples the repeater's four-wire circuit to another two-wire circuit such as the one in the central office.
The hybrid repeater's four-wire circuit has two separate signal transmission paths or portions, one being used to transmit signals in one direction from a first one of the term sets to the second one of the term sets, and the other being used to transmit signals in the opposite direction from the second term set to the first term set. Each term set separates the bidirectional signals, whereby signals in only one direction are transmitted over one of the four-wire transmission paths, and signals in only the opposite direction are transmitted over the other of the four-wire transmission paths. Each of the signal transmission paths or portions in the repeater's four-wire circuit is provided with an amplifier to thus amplify each direction of transmission separately.
Prior to this invention various forms of hybrid circuits have been proposed to accomplish the foregoing function in a repeater and, more generally, to provide an interface between a two-wire line, on the one hand, and a four-wire line on the other hand.
Perhaps the most commonly used hybrid circuit design is a hybrid transformer known as a hybrid coil. Although such hybrid transformers have been in use for many years, they suffer from the disadvantages of comparatively large size and high cost, among other things. To avoid these problems, various forms of transformerless or electronic hybrid circuits have been proposed such as the ones described in U.S. Pat. No. 4,064,377 issued on Dec. 20, 1977, U.S. Pat. No. 4,053,722 issued on Oct. 11, 1977, and U.S. Pat. No. 3,855,430 issued on Dec. 17, 1974. Prior amplifier circuits used to establish current sources are described in U.S. Pat. No. 3,870,896 issued on Mar. 11, 1975.